The present invention relates to a cooling apparatus for an internal combustion engine for executing a diagnosis of an operating state of a thermostat and a diagnosis method for the cooling apparatus
In the internal combustion engine, there can be generated a phenomenon (stuck-open valve) that a valve within the thermostat is not operated in an open state. In the state in which the stuck-open valve is generated, since coolant is always circulated via a radiator, a temperature of the coolant is hard to be increased in comparison with a normal state of the thermostat.
Accordingly, in a conventional cooling apparatus including a cooling apparatus described in Japanese Laid-Open Patent Publication No. 2000-220456, the configuration is made such that an abnormality of the thermostat is detected such as the following items (A) and (B) while paying attention to a difference of a temperature transition of the coolant between a normal state and an abnormal state of the thermostat.
(A) A reference temperature corresponding to a coolant temperature at a time when the thermostat is normal is calculated on the basis of a parameter having a correlation with the cooling temperature. Further, at a time of calculating the reference temperature, a vehicle speed having a correlation with a relative wind is added while taking into consideration a fact that the coolant temperature is affected by the relative wind.
(B) When the diagnosis condition is established, an operating state of the thermostat is diagnosed through a comparison between the reference temperature and an actual cooling temperature. That is, when an ascending degree of the reference temperature is larger than an ascending degree of the coolant temperature, it is determined that an abnormality is generated in the thermostat.
In a cooling apparatus described in Japanese Laid-Open Patent Publication No. 2004-316638, the configuration is made such that the influence of the relative wind applied to the coolant temperature is reflected on the reference temperature by correcting the reference temperature on the basis of the vehicle speed.
However, it has been confirmed through a test or the like executed by the present inventors that the affecting degree of the relative wind applied to the coolant temperature is greatly different between a case in which the traveling state of the vehicle is in a slope descending state and other traveling states. Accordingly, in the cooling apparatus in Japanese Laid-Open Patent Publication No. 2004-316638, in the case that the slope descending state of the vehicle is continued for a comparatively long period before the diagnosis condition is established, there is a possibility that the reference temperature is largely deviated from an essentially set value, and an abnormality of the thermostat is erroneously detected.